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Union of Filesystems

If you have ever used Unetbootin to create a bootable USB,
you may have noticed the option -

“Space used to preserve files
across reboots (Ubuntu only)”

What is different about Ubuntu? Actually, it is not so limited and
can be used for most distributions derived from Debian or Ubuntu.

On the other hand, if you use Fedora LiveUSB Creator, you
would have noticed the option:

Persistent Storage

Both Ubuntu and Fedora offer persistent storage but using
different mechanisms. I have found persistent storage immensely
useful, especially for booting from diskless machines. It is
interesting to learn about the differences between them and how they
work.

OverlayFS

If you try Unetbootin, it will create a file casper-rw on
the USB. In addition, an additional kernel boot option – persistent
– will be added in the syslinux configuration file.

The persistent storage need not be a file. It can be a partition
instead. The label of the partition has to be 'casper-rw'.
Persistence seems to work on a Mint live usb provided a partition is
used. For Mint,

Create two partitions a a usb

Install the live iso on one partition

Remove the casper-rw file

Format the second partition as ext2

Use e2label to change the label of the partition to
'casper-rw'

The root system will be mounted as 'overlayfs' type, which is a
module included in the Ubuntu/Debian derived distributions.

The system mounts the squashfs root as a readonly
filesystem(/rofs). It then mounts the casper-rw file/partition as a
read-write filesystem(/cow). Both are mounted as a composite
filesystem with /rofs as the lowerdir and /cow as the upperdir.
Overlayfs will search for a file on upperdir first and then on
lowerdir. If a file is modified, it will be stored in the upperdir.

Snapshots and Device Mapper

A layered filesystem is reminiscent of the description in
Snapshot(computer_storage) in Wikipedia. Volume Manager supports
snapshots. Fedora does not include the overlayfs module and uses the
concept of a snapshot to achieve the same goal.

As before, you will need a read only filesystem. However, the
overlay is no longer a filesystem. It, instead, stores information
about the blocks which have been changed and the modified data of the
block. So, the data read from the basic filesystem will be replaced
by data in the modified blocks.

This has some interesting consequences. A Fedora live usb also
contains a file containing a squash filesystem. When you loop mount
this file, you do not get a root filesystem. Instead, you will find
that it contains an image of an ext4 filesystem 'LiveOS/ext3fs'!
The reason for this convoluted setup becomes clear when you realise
that squashfs is a readonly filesystem. Hence, you cannot use it as a
read only snapshot and modify its data.

Fedora LiveUSB Creator will create a file with a name
LiveOS/overlay-LIVE-<UUID>. A kernel option –
rd.live.overlay=LABEL=LIVE - would be added in the syslinux
configuration file. A script in the initramfs will loop mount the
read only ext4 filesystem. It will create a volume manager snapshot
from it and then mount the snapshot with the overlay file as a
read-write filesystem on /dev/mapper/live-rw. The essentials
of the script will be as follows (adapted from dmsquash-live-root.sh
in /usr/lib/dracut/modules.d/90dmsquash-live/):

BASE=$( losetup -f )

losetup -r $BASE
/run/initramfs/squashfs/LiveOS/ext3fs.img

OVERLAY=$( losetup -f )

losetup $OVERLAY
/run/initramfs/<overlay-file-path>

sz=$(blockdev --getsz $BASE)

echo 0 $sz snapshot $BASE
$OVERLAY p 8 | dmsetup create live-rw

An obvious drawback of this approach is that you may use only a
filesystem supported by Volume Manager. This, in particular, rules
out network filesystems which can be useful in the context of LTSP
and similar diskless environments.

Unionfs/aufs

Unionfs and, later, aufs were designed to offer a union of
multiple filesystems. Each filesystem is stacked on top of the
preceding ones. The search for a file starts with the top stack and
goes down the stack. Each filesystem may be of a different type.
There are user space implementations of unionfs which use the fuse
kernel module.

Ubuntu switched from aufs to overlayfs. Overlayfs is simpler in
that it restricts the union to just two filesystems – an upper
layer and a lower layer. This module may be included in the 3.10
linux kernel.

Overlayfs mechanism is superb for packing a large set of
applications on a limited space by taking advantage of the
compression-enabled filesystem, like squashfs. You can go back to the
original state anytime by just clearing the upperdir. In addition to
being useful in a live usb, it is an excellent option for small
devices where the distribution of updates is controlled.